Synchronous frame current regulators are commonly used for current control of AC motors, such as three-phase electric motors. By providing dynamic control over a wide frequency range, synchronous frame current regulators are suited to many industrial applications. In digital implementations of conventional current regulators, as the ratio of the sampling frequency to the fundamental frequency, or synchronous frequency, of the AC motor decreases, the stability of these current regulators tends to decrease. For example, delays in digital implementation, increased sub-harmonics in voltage synthesis using pulse width modulation (PWM), or the like, tend to introduce instability.
To produce high torque within a limited volume, a high pole-count electric motor is useful, particularly for hybrid vehicle applications (e.g., hybrid electric vehicles or the like). An increased pole-count generally increases the fundamental frequency associated with the AC motor, while the switching and sampling frequency associated with the current regulation is generally limited due to limitations of the switching power device and the through-put of the processor. Typically, at maximum speed, the ratio of sampling frequency to fundamental frequency, fsamp/ffund, can be very small (e.g., smaller than a ratio of about ten (10)).
When this ratio is less than about ten (10), a discrete time domain controller may have a sufficiently pronounced influence on the synchronous frame current regulator. Furthermore, inner current loops associated with the current regulator may incur instability due to digital delays. Sub-harmonics associated with asynchronous PWM become significant when the ratio is lower than about twenty-one (21).
Accordingly, it is desirable to provide methods and systems for controlling an AC motor that stabilize current regulation at increased motor speeds. Additionally, it is desirable to provide methods and systems for current regulation of an AC motor that can operate with an extremely low ratio of sampling frequency to fundamental frequency. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.